The invention relates to a circuit arrangement for the contactless transmission of information about the temperature within a fuel assembly of a nuclear reactor, and particularly for the prevention of local breakdowns in the active zone thereof.
One of the most important problems of a safe operation of quick reactors is the detection of the start of so-called local breakdowns of a fuel assembly and their early liquidation (melt down). Local breakdowns of fuel assemblies are conditions wherein the permeability of the fuel assembly is gradually reduced, and thus the passage of liquid sodium is reduced and its outlet temperature increased. This condition can have different causes, such as the increase of the volume of the fuel in the course of burning, the clogging of the cross section of passages by dirt from the cooling medium, or the presence of foreign objects. In its consequences this process causes an overheating and later the melting of the fuel assembly. Due to heat exchange with the surrounding space there is a possibility of an avalanche extension of the breakdown to a certain zone, and in the limit case to the whole active zone. An immediate consequence is a long time shut-down of a nuclear power plant from operation and the threatening of the safety of people by released heat and intensive radiation.
It would be best possible to indicate the start of a local breakdown of a fuel assembly by measuring the rate of passage of the cooling medium in each fuel assembly. This method, however, meets substantial technical difficulties. The indication of the start of boiling of sodium by some acoustic method is explicit, but the following reaction of the breakdown system cannot prevent the destruction of the fuel assembly where the breakdown occurred. For these reasons, the best way for measuring temperatures seems to be the measuring of the temperatures at the output of each fuel assembly. A common introduction of this method is, however, prevented by the circumstance that the system for measuring temperatures must not be an obstacle for the charging and the exchange of fuel.
Actually known arrangements for the contactless transmission of information about temperatures from fuel assemblies of nuclear reactors employ an electromechanical transducer situated in the fuel assembly, which transducer transforms direct current of the thermoelement to pulsating current. A transformer winding within the fuel assembly is fed by this pulsating current, whereby the voltage produced by the thermoelement is transformed to a receiving winding outside the fuel assembly.
A drawback of this solution is the limited reliability of the mechanically movable parts of the electromechanical transducer under high temperatures, an unstable operation with variations of surrounding temperatures, variation of the resistance of contacts due to condensation of components released from insulating materials under high temperatures and variations of the parameters of springs which are used in the arrangement. The above-mentioned influences, including the influence of intensive radiation on aging and on the mechanical properties of materials, make the solution with an electromechanical transducer entirely out of question for a common application in nuclear energetics.